(1) Field of the Invention
The present invention relates to thermal delayed tack sheets which are non-tacky at ordinary temperature but which will show tackiness, when activated by heating and which can retain the tackiness for a certain period of time, even after a heat source has been taken off therefrom.
(2) Description of the Prior Art
Thermal delayed tack sheets are also called temperature-sensitive tack papers, and are described in, for example, "Adhesion Handbook", Polymer Publication Association, 12th Edition, Sept. 20, 1980. The thermal delayed tack sheets are sheets each having a coating layer composed basically of a polymeric material, i.e., an adhesive polymer, a plasticizer which is solid at room temperature and preferably a tackifier. Examples of the above mentioned polymeric materials include polyvinyl acetate, polybutyl methacrylate, vinyl chloride/vinylidene chloride copolymer, synthetic rubbers, vinyl acetate/2-ethylhexyl acrylate copolymer, vinyl acetate/ethylene copolymer, vinylpyrrolidone/styrene copolymer, styrene/butadiene copolymer and vinylpyrrolidone/ethyl acrylate copolymer. Examples of the above mentioned solid plasticizers include diphenyl phthalate, dihexyl phthalate, dicyclohexyl phthalate, dihydroabietyl phthalate, dimethyl isophthalate, sucrose benzoate, ethylene glycol dibenzoate, trimethylolethane tribenzoate, glyceride tribenzoate, pentaerythritol tetrabezoate, sucrose octaacetate, tricyclohexyl citrate, N-cyclohexyl-p-toluenesulfonamide, dehydroabietylamine carbonate and methoxyethyl stearate/urea complex. Examples of the tackifier include rosin derivatives (rosins, polymerized rosins, hydrogenated rosins and their esters with glycerine or pentaerythritol and the like, resin acid dimers and the like), terpene resins, petroleum resins, phenolic resins and xylene resins.
The polymeric material is the base of the adhesive and tack strength, and is called the adhesive polymer. The tackifier is an auxiliary component for increasing tackiness, when activated by heating, and it is preferably blended but is not an essential ingredient.
The solid plasticizer is solid at room temperature and thus does not provide the polymeric material with plasticizing properties at such a temperature level, but when heated, it will be melted, whereby the polymeric material will swell and melt, and will consequently show tackiness. After once melted, the plasticizer will crystallize slowly, and therefore, a tack retention time of the tack sheets after heat activation can be prolonged.
Further, a coating solution for the manufacture of the thermal delayed tack sheets can take the form a hot melt type where a tacky composition is heated and thereby melted and is then used for coating (e.g., Japanese Laid Open Patent Nos. 16055/1980 and 106984/1981), an organic solvent type where components are dissolved in an organic solvent and used for coating, and the organic solvent is then evaporated (e.g., Japanese Laid Open Patent No. 111246/1976), or an aqueous type emulsion with coating solution where an latex and water is used for coating and then dried (Souichi Muroi, "Polymer Latex Adhesive", Polymer Publication Association, June 10, 1984).